Flash Cache Page
Notes:
- Applies to Oracle 11g Release 2 and later.
- Applies to Oracle databases running on Solaris or Linux.
- The Flash Cache configuration parameter DB_FLASH_CACHE_FILE must be enabled and DB_FLASH_CACHE_SIZE must be set to greater than 0 to see flash cache data on the home page and in this drilldown.
The Flash Cache page shows statistics relating to the performance and use of flash cache. Flash cache acts as a secondary cache to the buffer cache. On databases that perform more reads than writes, flash cache may provide performance advantages. It is important to monitor the use of flash cache as in some systems it may reduce performance.
To open the Flash Cache page
- Select the Spotlight on Oracle connection in the Spotlight Browser.
-
Click Disk Storage | Flash Cache.
Flash Cache Statistics Chart
|
Data |
Description |
|---|---|
|
Inserts |
When a block is about to be flushed from the buffer cache, Oracle will attempt to insert the block into flash cache. This is the number of blocks per second successfully inserted into the flash cache. This value is taken from the statistic 'flash cache inserts' from V$SYSSTAT. |
|
Reads |
Number of blocks per second a session has read from the flash cache. This value is taken from the 'physical read flash cache hits' statistic from V$SYSSTAT. |
|
Insert skip: DBWR overloaded |
When a block is about to be flushed from the buffer cache and the DBWR is busy writing dirty buffers to disk, the flash cache insert is skipped. This is the number of blocks per second the flash cache insert skipped due to the DBWR being overloaded. This value is taken from the 'flash cache insert skip: DBWR overloaded' statistic from V$SYSSTAT. |
|
Insert skip: other |
When a block is about to be flushed from the buffer cache, but the flash cache insert is skipped due to other reasons (not because the DBWR is overloaded or because the block already exists in flash cache). This is the number of blocks per second the flash cache insert skipped due to other reasons. This value is the sum of the following statistics from V$SYSSTAT:
Tip: To see the values for these individual statistics, click the Statistics tab. |
|
Eviction |
The number of blocks per second evicted from the flash cache. The most common reason for blocks to be evicted from the flash cache is because the block was modified. This value is the sum of the following statistics from V$SYSSTAT:
Tip: To see the values for these individual statistics, click the Statistics tab. |
|
Physical reads |
The number of physical reads from disk. This value is taken from the 'physical reads' – 'physical read flash cache hits' statistic from V$SYSSTAT. |
Flash Cache Times Chart
|
Data |
Description |
|---|---|
|
DB time |
Amount of time sessions are active on the database. This value is taken from the statistic 'DB time' from V$SYS_TIME_MODEL. |
|
Single block flash cache read wait |
Amount of time spent doing single block reads from flash cache. This value is taken from wait event 'db flash cache single block physical read' from V$SYSTEM_EVENT. |
|
Multi block flash cache read wait |
Amount of time spent doing multi block reads from flash cache. This value is taken from wait event 'db flash cache multiblock physical read' from V$SYSTEM_EVENT. |
|
Flash cache write wait |
Amount of time the DBWR spent waiting on flash cache write to complete. Consider turning on asynchronous I/O using the 'filesystemio_options' parameter. This value is taken from the wait event 'db flash cache write' from V$SYSTEM_EVENT. |
|
Flash cache write complete wait |
This metric indicates that sessions are trying to update a block that is currently being written to flash cache. A high rate of flash cache write complete waits means that writing to the flash cache is holding up sessions from performing updates. This value is taken from wait event 'write complete waits: flash cache' from V$SYSTEM_EVENT. |
|
Buffered IO wait |
Amount of time spent reading from physical disk into the buffer cache. This value is the sum of the following wait events from V$SYSTEM_EVENT:
|
Top Sessions by Flash Cache I/O Grid
|
Column |
Description |
|---|---|
|
SID |
User session ID. |
|
Oracle User |
Oracle database account of the started session. |
|
Program |
Program running the session. |
|
Module |
Currently executing module name set by DBMS_APPLICATION_INFO.SET_MODULE. |
|
Flash waits/s |
Number of reads from flash cache per second. |
|
IO waits/s |
Number of reads from disk per second. |
|
Flash reads (s) |
Total time spent doing reads from flash cache. |
|
IO waits (s) |
Total time spent doing reads from physical disk. |
|
Time saved (s) |
Time that it would have taken to read blocks from physical disk that were actually read from flash cache. This value is calculated by taking the difference in latency for disk reads compared to flash cache reads, and multiplying it by the number of flash cache reads. |
|
DB time (s) |
Total amount of time the session was active. |
|
Flash read waits |
Total number of reads from flash cache. |
|
Flash cache latency (µs) |
Time, in microseconds, taken to read a single block from flash cache. |
|
Disk latency (µs) |
Time, in microseconds, taken to read a single block from physical disk. |
|
Block changes |
Total number of block changes the session has performed. |
|
CPU time (s) |
Amount of time spent using CPU. |
|
Logical reads |
Number of logical reads per second in the past sampling interval. This includes all requests for database blocks, irrespective of whether they were found in the database cache. |
|
Machine |
Name of the computer on which the client is running. |
|
Multi block flash read waits |
Total number of multi block reads from flash cache. |
|
Multi block flash read waits (s) |
Total time spent doing multi block reads from flash cache. |
|
Multi block flash read waits/s |
Number of multi block reads from flash cache per second. |
|
OS user |
Operating system user for the client. |
|
Physical reads |
The number of physical reads initiated by the session. |
|
Serial # |
Serial number of the session. SIDs can be reused after the session disconnects, but the combination of SID and Serial number is always unique. |
|
Server PID |
Operating system process ID for the Oracle server process. |
|
Single block flash read waits |
Total number of single block reads from flash cache. |
|
Single block flash read waits (s) |
Total time spent doing single block reads from flash cache. |
|
Single block flash read waits/s |
Number of single block reads from flash cache per second. |
Top Segments by Flash Cache I/O Grid
|
Column |
Description |
|---|---|
|
Object |
Object owner and name. |
|
Flash cache reads |
Number of reads from flash cache (optimized physical reads). |
|
Physical reads |
Number of reads from physical disk (physical reads - optimized physical reads). |
|
Flash cache blocks |
Number of blocks in the flash cache. |
|
Buffer cache blocks |
Number of blocks in the buffer cache. |
|
Total cached blocks |
Total number of blocks in both caches. Note this is not the addition of the last two columns as some blocks could be in both caches. |
|
Total blocks |
Total number of blocks on physical disk for the object. |
|
% blocks cached |
Ratio of total cached blocks to total blocks. |
Flash Cache Statistics
The Flash Cache Statistics grid shows statistics relevant to flash cache from V$SYSSTAT.